Recently, the offshore wind industry has grown at a rapid rate due to the higher wind speeds found at sea. However, due to the nature of the water and the weather conditions, the assembly and installation of offshore wind turbines has proved to be difficult and expensive. In order to reduce the cost and simplify the installation, some offshore wind turbines are being essentially fully assembled onshore, transported to the offshore site and then connected to the pre-installed offshore foundation. These pre-installed foundations would be constructions which are built into the sea-bed such as a monopile, jacket, tripod or suction caisson foundations.
Specialist vessels are required to transport the pre-assembled wind turbines and to upend them at the offshore site. Offshore cranes or other specialist lifting devices are then required to lift the wind turbine and lower it onto the pre-installed foundation. The need for specialist vessels, lifting devices and offshore cranes adds significantly to the overall expense of an offshore wind farm. The largest expense is the cost of the pre-installed offshore foundations and their installation.
For this reason, the offshore wind turbine industry is now moving towards floating turbines which can be developed to be essentially fully assembled onshore and transported upright to the site in order to be anchored. This is an improvement on the high costs and lengthy installation of an offshore foundation. The key feature of a floating wind turbine is the support structure. The main floating wind turbine support structures can be categorized into three groups: spar buoy, semi-submersible and TLP.
All three groups deal with the problem of stabilizing the floating wind turbine against the horizontal, vertical and rotational movements to which it is subjected after installation.
A semi-submersible is a partially submerged structure that is stabilized by the buoyancy of watertight containers that are fully submerged in the water. However, this means that semi-submersibles rely on a large water-plane area to stabilize the structure against the changing loads such as those resulting from the operation of the wind turbine. Therefore the dimensions of the semi-submersibles are very large.
A spar buoy structure for a floating turbine is stabilized by ballasting. The spar relies on a ballasted deep-draft hull to stabilize the floating wind turbine. This requires deep water depths of over 100 m. The structures are also very heavy and costly with the possibility of reduction in weight being highly unlikely. Due to the length of the spar structure, onshore assembly of the full turbine and support structure is probably not possible which increases the cost further due to the offshore cranes, lifting devices or specialist transport vessels required.
With regard to a TLP structure, vertical motions are eliminated by the tension-leg mooring system in which the tendons are anchored to the sea bed. This stability provided by the tendons, allows the platform size to be significantly reduced and it is therefore lighter and less expensive. In addition the structure can be fully submerged in order to reduce wave loads.
The most versatile and cost-effective of the support structures mentioned before is the TLP structure. The complete TLP system is such that the support structure is smaller and much less expensive than a semi-submergible and a spar buoy. However, the biggest problem involved is in the cost and complexity of transportation and installation of the TLP structure wind turbine due to its instable nature outside of its installed form which means that without anchoring the floating wind turbine to the sea bed, wind and water forces have a drastic impact on the buoyancy stability of the TLP based floating wind turbine.
Principally, there are two obvious solutions for the installation of a wind turbine with a TLP support structure, known from the state of art:
The first solution is to transport and install the whole TLP structure, namely the support structure, the tendons, the anchors, the wind turbine and the tower, all separately. However, the cost and amount of time required for this solution are considerably large.
The second solution is to make the TLP support structure larger and more complex in order for it to be able to support a fully assembled wind turbine during transportation by towing it to the installation site. However, this defeats the purpose of a TLP structure and would be closer in design and function to a semi-submersible structure.
In addition to floating wind turbines mentioned above, there is a type of offshore foundation that is classed as a gravity base foundation which also offers some of the advantages such as onshore assembly and shorter installation times at the site. This type of foundation generally does not require any drilling into the sea-bed or pre-installation and therefore the transportation and installation of wind turbines with gravity base foundations can essentially be compared to that of floating wind turbines.
There are existing examples of the transportation and installation of support structures for floating wind turbines and essentially fully assembled offshore wind turbines, for example the “Blue H” (TLP), Cowi (gravity base foundation) and GBF (gravity base foundation). These structures are known to the skilled person, however, they have several disadvantages, e.g. very time-consuming and labor-intensive installation, bulky and difficult to handle transportation means, large dimensions due to the level of support required of the structures, solely compatible with the support structure for which they are designed, very high installation costs. In addition, the known state of the art comprises installation or transportation structures made up of several separate units that do not form a single, self-supporting and independent structure.